The biology of the Grey-headed Gull was studied between 2004 and 2005 in South Africa's Gauteng, KwaZulu-Natal and Northern Cape provinces. Grey-headed Gulls have a widespread but patchy distribution in South Africa, occurring both inland and at the coast. Their largest population is centred on Gauteng, where the species appears to be a relatively recent colonizer and where the current breeding population is estimated at 2185 breeding pairs (the largest in South Africa). There is evidence that the species has also increased in other parts of South Africa, especially at Port Elizabeth in the Eastern Cape. The majority of coastal birds are found in KwaZulu-Natal and there is strong evidence for regular movements of adult birds between Durban and Lake St Lucia. By contrast, little evidence was found for a putative large-scale, regular movement between Gauteng and KwaZulu-Natal. The breeding biology of the Grey-headed Gull was studied at four sites in Gauteng and at Lake St Lucia's Lane Island during 2004 and 2005. The distance between the Gauteng sites ranges from 1.7 km, between Lakefield Pan and Korsman's Bird Sanctuary, to 25.3 km, between Bonaero Park and Modderfontein Pan. The approximate distance between Gauteng and Lake St Lucia is 460 km. The mean clutch size at all sites was 2.42 eggs and the mean incubation period was 24.9 days. Parental investment in incubation was approximately equal between the sexes while males spent more time in attendance and participated in more aggressive encounters. Empirical growth curves are given for mass, wing, culmen, head and foot morphometrics of Grey-headed Gull chicks. Intraspecific variation in breeding parameters reveal significant differences between sites, including: highly synchronous laying at Lake St Lucia; the largest eggs and fastest growing chicks at Gauteng's Modderfontein Pan (a small, peripheral colony); and the smallest eggs and slowest growing chicks at Gauteng's Lakefield Pan (a large, 'core' colony). Possible reasons for these differences include the relative localities of each site in terms of feeding opportunities, high levels of predation by African Fish Eagle's at Lane Island, and density dependent factors operating on the large colonies within the core population on Gauteng's East Rand. Overall daily egg survival was comparatively high for all sites in Gauteng and low for Lane Island nests. Morphometric, plumage and bare-parts data from a sample of trapped and resighted birds are used to age, sex and determine the timing and duration of moult in the Greyheaded Gull. Six age classes were identified and, for all measurements, males were significantly larger than females. The mean duration of primary moult was 136 days between October and January and there were two waves of secondary moult. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2006.